If html works for you, then there's no particular reason you should use anything else - unless ... it might matter that:

PDF files on the web usually are much more convenient for visitors to your web site who may want to save information they find there.

IF you put up a "document" that you expect others to read and perhaps take with them for future reference, html gives you virtually no control over what the document looks like on their browser, since they can choose their own fonts - which may or may not have all the characters you include in the document, they can put funky backgrounds on it, change the colors, etc.

If a user "saves" an html page that has any embedded objects such as graphic headers, images, etc., the user gets a file with the name of the page and a separate folder with the same name that contains all the linked objects. Since the links are local links, the user cannot change the name of the file to make it more accessible on his own machine except by opening it in a browser and doing a "save as" with a new file name. First - this doesn't always work, and Secondly it requires the user to keep track of two copies of your page. Any "file management" such as copying to a different folder or different drive requires that the document and it's associated folder full of objects be kept together in exactly the same relationship as existed for the first save. Files saved as html with accompanying folders full of linked objects tend to be rather brittle, and break easily.

Because saved html files are a pain in the netherend to manage, I usually resort to copy and paste. When you encourage copy/paste notes on your work, you encourage the taking of bits and pieces, possibly out of context.

If you provide the "document" as a PDF, everything needed to display the document is contained in a single file, that can be moved and/or renamed freely. If you embed the fonts used, you know EXACTLY what the document will look like on the user's machine. You also know that the user who saves it will save the entire document. You can specify whether a user is allowed to save it, whether and to what extent a user can edit it, and whether the user can or cannot extract bits and pieces of it to use in other programs.

If you only want to be sure your file "looks nice" when someone saves it, it would be courteous and thoughtful of you, and usually pretty simple, to link a Word .doc that they can save. If you want any control over whether they can easily mangle what you allow them to save, a PDF gives you, as the creator, much more control over whether they do only the things you are willing to have them do.

Even though the files are often a little larger, and downloads may take a little longer, when I find something I'd really like to keep in my own ref files, I greatly appreciate being offered a PDF. It's immaterial for sites that have little content that's interesting to me, of course, or for "short pages" that are easily copied and pasted into Word.

John